An image forming apparatus scans a document image, forms a developing agent image corresponding to the scanned image on a sheet and fixes the resultant image to the sheet by a fixing apparatus.
The fixing apparatus has a heating roller and pressing roller, and a developing agent image bearing sheet is passed between the heating roller and the pressing roller to fix the developing agent image to the sheet to the sheet. A tungsten halogen lamp, for example, is held inside the heating roller. The temperature of the heating roller is raised by the heat generated by the halogen lamp heater, and the developing agent on the sheet is melted under the heating of the heating roller.
In an induction heating type fixing apparatus, a coil for induction heating is held inside the heating roller and, by supplying high frequency current to the coil, a high frequency magnetic field is generated from the coil. Under the high frequency magnetic field, an eddy current is generated from the coil and, due to the Joule heat generated by the eddy current, heat generation occurs in the heating roller.
A heating roller for holding a halogen lamp heater or an induction heating coil is greater in its heat capacity. For such a heating roller of a greater heat capacity, a longer time is taken from after a start operation until the heating roller reaches a temperature necessary for a fixing process.